This invention relates to the continuous, long term treatment and containment of municipal solid waste such as incinerator ash and similar waste materials containing possibly elevated levels of heavy metals, and to landfill containment disposal cells and methods. The primary metals of concern are those which are identified by the EPA as having particular contaminant levels for inorganic chemicals, known as the eight "priority" metals. These are arsenic, cadmium, barium, silver, lead, mercury, chromium and selenium.
Cationic treatment of waste materials according to this invention involves the use of natural occurring cationic active material, that is, a material with high cationic exchange (CEC) value, placed in one or more layers in a disposal cell for the purpose of immobilizing the metals of concern, particularly lead, cadmium and mercury by a combination of CEC and pH. The invention is a viable solution to the concern that toxic metals may leach from the waste over hundreds or thousands of years in the cell and find their way through the underlying protective liner system into the groundwater aquifer.
The scientific principle of cation exchange is widely documented and accepted. The knowledge that certain natural occurring substances and soils have the power to exchange cations with solutions containing other cations was the outgrowth of observations dating back to the remote past. For example, it was known for centuries that liquid wastes become decolorized and deodorized when filtered through solids. For many years the term "base exchange" was used to describe the reaction, even long after it was established that the hydrogen ion may take part in the exchange reaction. In early experiments it was shown that when soils were mixed with ammonia and then leached with water, the greater part of the ammonia was held back.
The cationic exchange capacity (CEC) of any naturally occurring soil or substance is usually expressed in units of milliequivalents per 100 grams of soil or meq/100 g. A number of naturally occurring coals and soils are known to possess this property. The exchange mechanisms of lignite have been investigated since the 1930's. Lignite has high cation exchange capacity due to its clay and high organic content. These are known to increase CEC values in some soils up to levels as high at 500 meq/100 g.
Lignite removes metals from leachate by two principal mechanisms. Among the organic compounds in lignite are several that immobilize metals by complexing or chelating them. However, the principal capacity of lignite to immobilize metals is of ionic exchange. When cations in leachate encounter negatively charged sites or surfaces, the dissolved cations change places with cations already adsorbed at these sites. This is called cation exchange. Certain cations are preferentially exchanged, among them mercury, lead and cadmium. By this mechanism, metals of concern in MSW ash leachate can be adsorbed and immobilized by negatively charged functional groups and surfaces of lignite and thus removed from the leachate.
The selection order for adsorption of cations (Cationic Exchange Mechanism) by clays is as follows: EQU Hg&gt;Pb&gt;Zn&gt;Cd&gt;Si&gt;Mg&gt;K&gt;Na
According to researchers, cadmium and zinc usually share an intermediate adsorption position when seven types of soil were tested.
The preferential selection order for cation adsorption in lignites is similar to that of clays, due to the high clay content and organic matter content of lignites. The following equation is an example of how a Cu+ ion in the water phase is attracted to a clay surfaces: EQU [Soil] Ca2++Cu2+.fwdarw.[Soil] Cu2++Ca2+
A similar equation for lignite would be expressed as follows: EQU [Lignite coal] Ca2++Pb2+.fwdarw.[Lignite Coal] Pb2++Ca2+.
As the CEC for a particular material increases, its capacity to immobilize trace metals that are preferentially adsorbed also increases.
In spite of extensive knowledge as to the ability of certain natural occurring soils and substances, such as clays, lignite, low rank brown coal and peat to immobilize metals, there seems to have been little effort to use these materials in a controlled manner in landfill cells, for the long-term containment of heavy metals to protect underlying and contiguous ground aquifers.